As is commonly known to persons skilled in the art, an optical storage disc comprises at least one track, either in the form of a continuous spiral or in the form of multiple concentric circles, of storage space where information may be stored in the form of a data pattern.
For writing information into the storage space of the optical storage disc, or for reading information from the disc, an optical disc drive comprises, on the one hand, rotating means for receiving and rotating an optical disc, and on the other hand optical means for generating an optical beam, typically a laser beam, and for scanning the storage track with said laser beam.
For optically scanning the rotating disc, an optical disc drive comprises a light beam generator device (typically a laser diode), an objective lens for focussing the light beam into a focal spot on the disc, and an optical detector for receiving the reflected light reflected from the disc and for generating an electrical detector output signal.
During operation, the focal spot should remain aligned with a track or should be capable of being positioned with respect to a new track. To this end, at least the objective lens is mounted radially displaceable, and the optical disc drive comprises radial actuator means for controlling the radial position of the objective lens.
The electrical detector output signal contains information on the tracking error, i.e. the radial distance from the centre of the focal spot to the centre of the track being followed. This electrical detector output signal is received by a control circuit, which processes the electrical detector output signal in order to generate a control circuit for the radial actuator.
One well-known method to process the electrical detector output signal is to generate a push/pull signal. The push/pull method has some disadvantages.
One disadvantage of the push/pull method is the sensitivity to beamlanding errors, i.e. a displacement of the light spot with respect to the optical detector.
A well-known solution to this problem is the three-spot push/pull method. Although the three-spot push/pull method solves the beamlanding-sensitivity problem of the one-spot push/pull method, it introduces other disadvantages. For one, it is necessary to use hardware equipment for generating three spots, i.e. a three-spot grating, which also needs to be aligned; this adds to the complexity and costs of the optical system. Further, a three-spot grating effectively splits one laser beam into three, namely one main beam and two auxiliary beams, resulting in a reduced light intensity of the main beam.
U.S. Pat. No. 6,388,964 discloses a tracking method where a tracking error signal is generated from the detector output signal on the basis of a differential phase detection method. The method as described in this document applies to ROM-drives, i.e. applies to discs containing data. This means that the method as disclosed in said document can not be applied directly in a drive suitable for handling writable discs, because writable discs may have tracks without data.